I. Field of the Invention
The present invention is directed to curable two-part laminating adhesive compositions useful in laminating flexible films and to a method for the preparation thereof. The adhesive compositions are prepared from isocyanateterminated urethane prepolymers which have been converted via diketimine intermediates and alcoholic solvents to controlled chain extended backbones containing ketimine and/or amine functional groups. The presence of a portion of the amine groups in the form of ketimine groups prevents gelation during production while providing a source of additional amine groups for subsequent reaction and curing with epoxy resins. Flexible film laminates formed using these adhesives are characterized by rapid curing, high bond strength, superior heat resistance, both chemical and water resistance. These laminates therefore find application in a wide variety of flexible packaging applications.
II. Brief Description of the Prior Art
The packaging industry, particularly the food packaging area thereof, is currently utilizing large quantities of flexible films. Since all the properties desired in such films are not available in any one film, the industry generally employs laminates prepared from a combination of films. Most often, these laminates are formed from Saran, or Saran coated substrates, Mylar, nylon film or paper laminated onto a film or sheet of a corona treated polyolefin.
In forming such laminates an adhesive is first coated onto one of the film webs. The coated web is thereafter heated to remove any solvent from the adhesive and then heat-nipped to the other film web. Since the time of exposure of the adhesive to heat either in the drying oven or heat nipping machine, is very brief, it is necessary to employ an adhesive which will cure at room temperature after the laminate has been formed. Moreover, the resulting laminate seal must develop or cure into a strong bond with no delamination occurring even when exposed to the frictional stresses and elevated temperatures encountered during formation of bags or containers therefrom, or to water or chemicals such as may occur during filling of the containers or during use and storage thereof.
Heretofore, the most satisfactory laminates as indicated by industry acceptance have been formed using urethane-based isocyanate prepolymer adhesives. Most of these urethane adhesives, however, have the disadvantage of requiring organic solvents such as methyl ethyl ketone, ethyl acetate, toluene or acetone which are unreactive with the NCO group in order to form an applicable solution. Such solvents must have extremely low moisture contents to prevent instability, they are further subject to variable humidity and environmental conditions which affect the stability, pot-life and cure properties of the adhesive system during application. Moreover, the necessity for the evaporation of huge quantities of volatile solvents from the adhesive film during curing is undesirable because of the increasing awareness of the deleterious effects of various contaminants on our environment.
In an attempt to minimize this moisture curing variable and simultaneously eliminate the dependence on expensive, unreactive, low moisture content solvents, efforts have been made to utilize the more acceptable, readily available, moisturetolerant solvents such as low alkyl alcohols together, if necessary, with much smaller amounts of acetone and methyl ethyl ketone.
In order to accomplish this end, it has been necessary to convert the terminal NCO portions of the polyurethane prepolymer chain to another functional group which is unreactive with the proposed water tolerant solvent systems. It has been found that the conversion of the NCO group to an amine group is most satisfactory for such purposes since it will react with the postadded polyepoxides used for curing so as to form satisfactory adhesives. However, the direct addition of liquid aliphatic polyamines such as diethylene triamine yields uncontrolled localized rapid reactions with the NCO groups resulting in varying degrees of gelation. Thus, U.S. Pat. No. 3,565,972 describes the use of solid aliphatic or aromatic diamines (preferably in latent form) together with a small amount of a Lewis acid-amine complex which cures on exposure to heat.
Additionally, U.S. Pat. No. 3,931,116 discloses a two-part adhesive composition wherein the amine terminated polyurethane urea polymer is formed by the addition of the isocyanate terminated urethane prepolymer into excess diamine, preferably cycloaliphatic diamines (which are possibly chosen for their steric hindrance so as to control the undesirable and excessive chain extension inherent in this step). This latter approach, however, suffers from some serious disadvantages. Since the diamine cannot be added directly to the urethane prepolymer, the process necessitates the use of two reaction vessels in the preparation of the adhesive, a costly and time-consuming disadvantage. Moreover, although the initial green strength is adequate (over 500 psi), the final cured bonds never attain the tear property required of commercial laminating adhesives due, most likely, to the inherent stearic hindrance of the initial amines and their low order of reactivity with the post-added polyepoxides.
It is therefore an object of the present invention to provide a laminating adhesive composition suitable for use in the formation of film laminates for the flexible packaging industry. It is a further object to provide such an adhesive which, when used to form a laminate, will result in one characterized by superior bond strength. It is also an object of the invention to provide an adhesive which is tolerant to moisture and which may be prepared and utilized in low cost, readily available alcoholic solutions. Finally it is an object of the invention to provide a method for the production of such adhesives at lower costs where one kettle processing techniques may be employed. These and other objects will be apparent from the description which follows.